Timeline - updated

@migration #planning #timeline #importer #plugin #cli #xmlrpc

Since I wasn't aware of the 2+ week bonding/orientation period at the start of the GSoC program, my original proposal had a timeline that started off too early. Here, I've updated it to push things back to align with the GSoC program structure. This also accounts for changes to the scope of the project to focus on the importer/exporter instead of file/DB migration. (I will write a separate justification for this change in scope.)

The long version

Note: when I refer to implementing something in a plugin, I generally mean that the actual functions will be abstracted from the way it's hooked into WordPress, to make it possible for certain features to be incorporated elsewhere (especially WP CLI and core).